With the increasing popularity of communication devices, service providers have introduced a variety of services. To make these services available to users, service providers use a variety of network technologies. The particular network technology used generally depends upon the type of service being offered, and its technical requirement, e.g., the bandwidth required. Service providers have also implemented the capability of providing similar services over different network technologies, including cellular networks, as well as packet switched networks, such as wireless local area networks (WLANs). A user might wish to use one or the other type of network technology. As an example, voice calls made over a WLAN may be chargeable at a lower tariff rate than those made over cellular networks. Due to such differences, the user might wish to use a WLAN when available to make voice over Internet Protocol (VoIP) calls. Existing systems with a call server in a WLAN allow a call originating in the WLAN to be terminated in either a cellular network or the WLAN. However, a call originating in a cellular network typically cannot be terminated in a WLAN unless two-stage calling is used to anchor the call on the WLAN call server first. This is because a call originating in the cellular network normally communicates with a cellular network call server only, and does not go through the WLAN call server. Thus, it is difficult to seamlessly handoff a call originated in a cellular network to a WLAN.
One solution comprises the two-stage calling. For two-stage calling, when a user in a cellular network makes a call, the call is anchored at the WLAN call server when the call is originated. That is, the call is made through the call server and includes at least one leg in the cellular network between the caller and the WLAN call server. Then, the call is routed to the callee via the WLAN call server, for example, in the cellular network or in a public switched network (PSTN). The WLAN call server thus facilitates handoff to the WLAN when the user is in range of the WLAN. Two-stage calling, however, is not needed for most calls because most calls originated in the cellular network are completed or terminated before a user enters the WLAN. For example, many cellular calls may be short and have a duration only when the user is out of range from the WLAN. Also, resources at the WLAN call server may be limited, and routing all calls through the WLAN call server may overwhelm the WLAN call server and result in dropped calls or being unable to make calls when needed.